Conventionally, a rapid thermal annealer (RTA) is well known as a heat treatment apparatus for rapid heating and rapid cooling of a silicon wafer. As described in FIG. 3(a) of Patent Reference 1, a ring-shaped wafer holder 100 is mounted within a furnace of the rapid thermal annealer in order to maintain a horizontal orientation of the wafer.
As shown in Patent Reference 2, in another well known type of wafer holder, a wafer is supported by a plurality of points.
However, in the heat treatment of wafers horizontally supported during the treatment, there has been a problem of yield reduction caused by generation of slip dislocations. It is considered that, since supported portions of the wafer are pressed by their own weight, sliding friction between the wafer and the projections is caused by warping of the wafer during the heat treatment or by a difference in heat expansion, strain occurs in a portion of concentrated weight, and therefore each portion supported by support projections generates slip dislocations.
In conventional wafer holders, reduction of slip dislocation is devised. An art of Patent Reference 2 is directed to reducing slip dislocation caused by the presence of an orientation flat.
For reduction of slip dislocations in a wafer lacking an orientation flat, Non Patent Reference 1 describes that, regarding support positions of the wafer by a wafer holder having a plurality of support points, the support points are preferably positioned at a region where a radial distance from a center is defined by 80 to 85% of the wafer radius.    Patent Reference 1: Japanese Unexamined Patent Application, First Publication No. 2002-134593.    Patent Reference 2: Japanese Unexamined Patent Application, First Publication No. 2002-170865.    Non Patent Reference 1: Takeda, R. et al. J. Electrochem. Soc., Vol. 144, No. 10, October (1997), pp. L280-L282.
However, even a wafer lacking an orientation flat generates slips when the wafer is mounted on a wafer holder of a rapid thermal apparatus and heated rapidly under a furnace temperature of not less than 1000° C. There is still further demand for a greater reduction of such dislocations.